1. Field of Invention
This invention relates to control circuits. More particularly, the invention relates to control circuits for optical links using semiconductor lasers.
2. Description of Prior Art
When directly modulated semiconductor lasers are used in an analog link the pre-bias current is normally set so that the lasers average output power remains constant. This is done with a simple feedback circuit. Without such a circuit the laser's pre-bias current would need to be set high enough to insure that the laser's average output power was adequately high over the laser's lifetime, and over the operating temperature range. Such a high pre-bias current may cause the laser to output too much power when new, or when the laser temperature is low: excess optical power can saturate a receiver or induce fiber non-linearity, thus degrading system performance. Operating the laser at high current can also accelerate aging of the laser. Therefore, using a constant bias current may degrade system performance.
When this simple prior art feedback circuit is used with uncooled lasers (laser modules lacking temperature control), the pre-bias current will get lower at lower temperatures, and as the pre-bias current falls the link becomes more susceptible to clipping-induced errors. These errors occur when the laser current (the sum of the pre-bias current and the modulating signal) falls below the lasing threshold.
Prior art related to controlling the laser pre-bias current is as follows:
U.S. Pat. No. 4,009,385 issued Feb. 22, 1977, discloses a control circuit for an injection laser including a driver circuit which applies a driving current to the laser in response to an electrical modulating signal. A pre-bias circuit applies a pre-bias current to the laser in response to the difference between a signal derived from the electrical modulating signal and a signal derived from the laser light output.
U.S. Pat. No. 4,292,606 issued Sep. 29, 1981, discloses a circuit which senses the time intervals during which a total current applied to a laser diode exceeds a second break point value on the light-current characteristic curve and which generates an error signal that may be fed back to adjust a current modulator.
U.S. Pat. No. 4,484,331 issued Nov. 20, 1984, discloses a current regulator circuit using electrical feedback to stabilize the bias current of semiconductor laser diodes. Laser diode current is sensed with negative-temperature-coefficient resistance to develop voltages which are compared with zero-temperature coefficient voltages thereby generating error signals, which regulate emission from the laser diode against changes with temperature.
U.S. Pat. No. 4,903,273 issued Feb. 20, 1990, discloses a control circuit in which the operating point of the laser diode is positioned in the portion of the characteristic curve in which the ratio of the slope of the characteristic curve to the slope of the characteristic curve in the linear portion of the characteristic curve is equal to a predetermined value. This ratio is obtained by measuring a change in the bias current which compensates for power change caused by a change of the modulation current.
U.S. Pat. No. 4,958,926 issued Sep. 25, 1990, discloses a closed loop control system for a laser in which the output light of the laser and light from a fiber to which the laser light is coupled initially sets the bias and pulse current amplitude to the laser and adjusts the gain of the receiver which is part of the control system. In response to light from the fiber the control system continuously controls the pulse current amplitude and receiver gain so that the amplitude or the electrical signal representative of the light is kept within the operating range of the receiver electronics.
U.S. Pat. No. 5,073,983 issued Dec. 17, 1991, discloses an amplitude modulated optical signaling system having improved linearity and reduced distortion by detecting a portion of the optical output from the light emitter to obtain an electrical feedback and phase reversing a first portion of feedback signal and degeneratively combining it with the electrical input of the signal to obtain a combined signal for ac modulating the light emitter. The second portion of feedback signal is desirably time averaged and used to control the DC drive to a light emitter so that the average light output is substantially constant. The distortion otherwise introduced in the optical signaling system by the non-linearity associated with the optical emitters driven at high power levels is reduced.
U.S. Pat. No. 5,260,955 issued Nov. 9, 1993, discloses automatically setting a threshold current for a laser diode which has a linear laser operating region and a lower non-linear operating region. The laser diode is controlled (1) during a warm up period, during which no operating current is supplied to the laser diode, to allow the electronics and laser diode to stabilize; (2) during an initialization period, during which the current for the laser diode is brought up to a threshold operating point above said non-linear operating region and during which no image data is applied to the laser diode; and (3) during an operating period in which digital image data is applied to the laser diode to operate it in the linear laser operating region.
None of the prior art discloses a control circuit for a laser which (1) achieves minimum average laser output power and (2) prevents clipping induced errors in the laser output by forcing the average laser output power to exceed the minimum when the pre-bias current would otherwise fall below a minimum value.